Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard chamber to an advance chamber. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent (or Auto-lock), Retard, and Advance in the specified order. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. In the case of a cam torque actuated (CTA) mid-lock VCT phaser, when the spool valve is commanded to move into the retard or advance region, the detent circuit is prevented from “auto-locking” the cam phaser by the presence of high pressure oil going into the piloted valve. If either the spool valve is commanded to detent, or it is commanded to move into the retard, null, or advance region, and insufficient system oil pressure is present, the detent oil circuit may engage and attempt to hydraulically control the phaser back to the intermediate mid-lock position. If this occurs with low piloted valve oil pressure due to low system oil pressure, there may be a conflict between VCT position control and hydraulic control of the locking pin. Consequently, the phaser may be locked even when commanded to an advance or retard position. Alternatively, the phaser may not predictably respond to spool valve commands due to additional and erratic actuation via the flow of fluid through detent circuit lines. These problems may occur even when hydraulic pressure is sufficient to control the phaser via the phasing circuit lines. Either case may result in engine performance degradation.
In one example, the above issue may be at least partly addressed by a method for an engine, comprising moving cam timing via cam torque actuated hydraulic pressure separate from engine generated oil pressure via a spool valve and in response to system oil pressure falling below a threshold, adjusting a position of the spool valve to reduce the system oil pressure applied to a locking circuit of the phaser. In this way, inadvertent engagement and locking of the detent circuit can be averted.
As an example, an oil pressure of a VCT phaser may be monitored. During conditions when the oil pressure is below a threshold, the spool valve may be automatically commanded to the detent region so that the phaser may be directed to the mid-lock position and the locking pin may be engaged. In addition, commanding the spool valve to the advance or retard regions may be disallowed. The phaser may remain in the mid-lock position with the pin engaged until the oil pressure returns above the threshold pressure. In one example, if the oil pressure in the VCT phaser is low due to low bulk oil pressure, the engine controller may increase the engine idle speed to raise the bulk oil pressure sufficiently such that the VCT phaser oil pressure rises above the threshold. Once the oil pressure is sufficiently high, command of the spool valve to the advance or retard regions may again be allowed.
In this way, by actively commanding the spool valve to the auto-lock position responsive to low hydraulic fluid (e.g., oil) pressure, the spool valve may disallow VCT position controls to conflict with inadvertent engagement of the detent oil circuit due to the low oil pressure. Instead, the spool valve may only allow oil flow to be directed through the detent circuit until the oil pressure returns to the system. By intentionally engaging the detent circuit during low oil pressure conditions, the presence of competing oil flow through the phasing circuit lines is averted. Additionally, by commanding the spool valve to remain in the detent position until sufficient oil pressure returns to the system, inadvertent engagement of the detent circuit is avoided. Overall, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.